1. Field of the Invention
The present invention relates to a creasing device and an image forming system.
2. Description of the Related Art
What is called saddle-stitched or center-folded booklet production has been conventionally performed. The saddle-stitched booklet production is performed by saddle stitching a sheet batch, which is a stack of a plurality of sheets delivered from an image forming apparatus, and folding the thus-saddle-stitched sheet batch in the middle of the sheet batch. Folding such a sheet batch containing a plurality of sheets can cause outside sheets of the sheet batch to be stretched at a fold line by a greater amount than inside sheets. Image portions at the fold line on outside sheets can thus be stretched, resulting in damage, such as come off of toner, to the image portions in some cases. A similar phenomenon can occur when other fold, such as z-fold or tri-fold, is performed. A sheet batch can be folded insufficiently depending on the thickness of the sheet batch.
Creasing devices that crease (score) a sheet batch prior to a folding process where the sheet batch undergoes half fold or the like to make outside sheets easy to fold, thereby preventing come off of toner have already been known. Some type of such creasing devices produce a crease in a sheet in a direction perpendicular to a sheet conveying direction by moving a roller on a sheet, burning a sheet with a laser beam, pressing a creasing blade against a sheet, or a like method.
A known example of such a creasing device is disclosed in Japanese Patent Application Laid-open No. 2009-166928. A technique of moving a creasing member by using a plurality of individually-advancing-and-retracting mechanisms, which are activated at different times, so that the creasing member presses a sheet with a gradually-decreasing pressure to produce a crease is disclosed in Japanese Patent Application Laid-open No. 2009-166928.
However, producing a crease in a sheet with a roller involves moving the roller across a length of the sheet in a direction, along which a fold extends, and therefore is time consuming. This can be resolved by rotating the sheet conveying direction by 90 degrees and producing a crease parallel to the sheet conveying direction; however, this scheme involves a change in footprint and therefore is disadvantageous for space-saving design. Creasing by using a laser beam is environmentally less favorable because smoke and odor are given off during creasing.
Creasing a sheet by pressing a creasing blade against the sheet can be performed in a relatively short period of time and allows easy production of a crease perpendicular to a sheet conveying direction; however, pressing a longitudinal face of the creasing blade against the sheet entirely at once can increase a load. To reduce the load, a scheme of virtually dividing the face of the creasing blade into a plurality of portions and bringing the creasing blade face into contact with a sheet a plurality of times, one portion each time, can be used. However, this scheme is disadvantageous in that unevenness can develop between a portion that contacts the blade multiple times and a portion that contacts the blade only once and also in that producing a crease by making contact multiple times can decrease productivity.
To solve the inconveniences discussed above, it is possible to reduce a load placed on a creasing moving unit by bringing a creasing blade gradually into contact with a sheet from an edge of the sheet and causing a creasing unit to contact the sheet only once; however, this causes a pressure applied onto a center portion of the sheet to be weakened, making it difficult to produce an even crease.